[October, 
518 The Philosophy of the Radiometer 
collision must modify its motions ; and such modification 
under given conditions would form a fine subject for the ex- 
ercise of the ingenuity of molecular mathematicians. If 
their hypothetical data are sound, they should be able to 
predict the relative velocities or torsion force of a series of 
radiometers of similar construction in all other respeCts, 
but with variable shapes and diameters of enclosing vessels. 
If we divest our minds of all visions of hypothetical 
atoms, molecules, ethers, &c., and simply look at the 
faCts of radiation with the same humility of intellect as 
we usually regard gravitation, this primary difficulty of 
the radiometer at once vanishes. The force of gravitation 
is a radiant force aCting somehow between, or upon, or by 
distant bodies ; and these bodies, however far apart, aCt and 
reaCt upon each other with mutual forces, precisely equal 
and exactly contrary. We conceive the sun pulling the 
earth in a certain direction, and receiving from the earth an 
equal pull in a precisely contrary direction, and we have 
hitherto demanded no ethereal or molecular link for the 
transmission of these mutually attractive forces. Why, 
then, should we not regard radiant repulsive energy in the 
same simple manner ? 
If we do this there is no difficulty in finding the ultimate 
reaction fulcrum of the radiometer vanes. It is simply the 
radiating body, the match, the candle, the lamp, the sun, or 
whatever else may be the source of the impelling radiations. 
According to this view, the radiant source must be repelled 
with precisely the same energy as the arms or pendulum of 
the radiometer; and it would move backward or in opposite 
direction if equally free to move. If, by any means, we 
cause the glass envelope of the radiometer to become the 
radiant source, it should be repelled, and may even rotate 
in opposite direction to the vanes or vice versa. This has 
been done with floating radiometers. 
Viewed thus as simple matter of faCt, irrespective of any 
preconceived kinetics of intervening media, the net result 
of Mr. Crookes’s researches become nothing less than the 
discovery of a new law of nature of great magnitude and 
the broadest possible generality, viz., that the sun and all 
other radiant bodies, i.e., all the materials of the universe, 
exert a mechanical repulsive force, in addition to the calo- 
rific, luminous, aCtinic, and electrical forces with which they 
have hitherto been credited. He has shown that this force 
is refrangible and dispersible, that it is outspread with the 
speCtrum, but is most concentrated, or aCtive, in the region 
of the ultra-red rays, and progressively feeblest in the violet ; 
